Development of miniaturized electronic systems has driven the demand for miniaturized power sources that can be integrated into such systems. Several kinds of micron-sized power sources such as micro-batteries, micro-fuel cells, and energy harvesters have been developed in recent years. However, for the applications that require high power, there is a need for miniaturized electrochemical capacitors (micro-capacitors). Electrochemical micro-capacitors with high power density can be coupled with energy harvesting devices to store the generated energy. Moreover, they can also be paired with micro-batteries to provide the peak power and improve the cycle lifetime. Based on the charge storage mechanism, electrochemical capacitors (ECs) can be divided into electric double layer capacitors (EDLCs) and pseudo-capacitors. The former utilizes interfacial double layer capacitance of various types of carbon materials to store electric charge. The latter, the pseudo-capacitor or redox capacitor, uses fast and reversible surface or near-surface redox reactions for charge storage. The active materials of pseudo-capacitors include transition metal oxides and conductive polymers. Micro-capacitors of both types have been reported in the literature. For example, Lim et al., 148 J. Electrochem. Soc. A275-278 (2001) reported that a thin film EC based on pseudo-capacitive ruthenium oxide (RuO2) and Lipon solid electrolyte delivered a volumetric capacitance of about 38 mFcm−2 μm−1, however its capacitance dropped by 53% after 500 cycles.
Electrochemical micro-capacitor based on conductive polymer was first reported by Sung et al., 133 J. Power Sources 312-19 (2004) who fabricated Polypyrrole (Ppy) micro-electrodes by electrochemical deposition on interdigitated gold electrodes. More recently, Sun et al., 193 J. Power Sources 924-29 (2009) reported the fabrication of three dimensional (3D) Ppy electrode architectures for micro-capacitors with geometric capacitance of the 27 mFcm−2 (normalized by the footprint area) at 1 mAcm−2 current density. On the other hand, EDLCs usually have higher rate capability, higher power density, and an extended cyclic life compared to pseudo-capacitors. In recent years, there have been some efforts to fabricate micro-scale EDLCs. For example, fabrication of printable thin film ECs with single-walled carbon nanotubes as electroactive materials has been reported by Kaempgen et al., where the estimated capacitance of the fabricated cell was 1.1 mFcm−2, in a potential window of 0 to 1.0 V. In addition, ECs from inkjet printing of activated carbon powders on interdigitated gold current collectors reached the maximum cell capacitance of 2.1 mFcm−2 at a low scan rate of 1 mVs−1.